An action that occurs during anal sex where the female has been pushed nearly upsidown with her anus pointing upwards, the rectum opens letting air flow in and whilst the male is thrusting the air is forced out creating a false fart. The moment where the air is inside the rectum is known as a bum balloon, it is similar to a Qweef.

Gavin: I was having anal sex with my girlfriend last night and she farted!
James: Sure she didn't have a bumballoon?